Web tensioning device

ABSTRACT

A web tensioning device utilizes a dancer arm which is pivoted by a servo motor controlled in a feedback mode. The controller for the servo motor receives an input signal based on the acceleration (positive or negative) of the dancer arm and implements a position adjustment necessary to maintain predetermined web tension.

FIELD OF INVENTION

[0001] This invention relates to devices for tensioning webs, such aspaper webs or textile webs, during processing.

BACKGROUND OF THE INVENTION

[0002] Web tension control is important in paper conversion processesand the like. In high speed operations, even relatively small variationsin web tension may cause web processing difficulties downstream.Moreover, in relatively high speed web lines, such as those exceedingabout 500 feet per minute (about 150 meters per minute), manualadjustment of web tension is not practical.

[0003] In web tensioning systems where web tension is controlled ormaintained by a dancer, a limitation on effective control of web tensionis the response time of the dancer inasmuch as often the dancer movementlags behind the actual changes in web tension. A primary cause of suchrelatively long response time is the weight of the dancer. Thus, itwould be desirable to have a web tensioning device that responds readilyto variations in web tension with minimum lag time. The presentinvention satisfies this need.

SUMMARY OF THE INVENTION

[0004] A web tensioning device of the present invention provides animproved dancer arm and a controller therefor that responds readily tovariations in web tension during web processing with minimal delay, andmaintains web tension within predetermined limits without reliance ongravitational forces.

[0005] The present web tensioning device includes a base, a dancer armwith a dancer and pivotably mounted to the base, and a servo motor thatpivots the dancer arm in response to a control output signal whichdepends on web tension requirement at a given point in time. The controloutput signal to the servo motor is provided by a controller that isoperably associated with the servo motor and is responsive to a webtension requirement communicated to the controller either by movement ofthe dancer arm or a web feed rate actuator.

[0006] The dancer can be a dancer roller, a fixed shaft, an air bearingfixed shaft, and the like.

[0007] In one particular embodiment, the present web tensioning deviceincludes a dancer arm suitable for engaging the web to be tensioned. Thedancer arm has a free end portion that carries a dancer roller mountedthereon and a fixed end portion that is pivotably mounted to a base andso as to coact with an angular position sensor that indicates therelative angular displacement of the dancer arm while tension ismaintained in the web. The web can be a paper web fed to a paperconverting machine, a fabric web, and the like.

[0008] A servo motor is operably associated with the dancer arm forpivotally positioning the dancer arm in engagement with the web inresponse to a control signal from a controller. Input to the controlleris provided by the angular position sensor, a position feedback devicesuch as an encoder, that monitors the angular displacement of the dancerarm from a predetermined position as the web is fed from an unwind rollto further processing station or stations.

BRIEF DESCRIPTION OF THE DRAWING

[0009] In the drawing, FIG. 1 is a schematic depiction of the webtensioning device embodying the present invention and utilizing aposition feedback mode of operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] An industrial web converting machine usually performs itsprocessing function utilizing a continuous sheet or web of paper orother non-woven material (e.g., yarn, wire, tubing, or filament),fabric, foil, and the like. The web is pulled by one or more powerdriven rollers through a series of idler (or non-driven) rollers to oneor more processing stations or stages where the web is folded, cut intosegments, shaped, etc. The web is fed into the machine from a relativelylarge roll called an “unwind roll” which rotates on a roll stand.

[0011] The web often travels at different speeds through differentsections of the converting machine and becomes slack from time to time,either by design or otherwise. A web that is slack can becomeundesirably tangled or wrinkled. To control slack, a web processingsystem frequently utilizes a dancer which typically is one or more idlerrollers or shafts on non-rotating arms that move freely against the web,usually by being mounted on a pivoting arm of some sort. The dancercontrols web tension by increasing or decreasing at any point in timethe distance traveled by the web. Under balanced conditions the dancerremains stationary, i.e., in a neutral position, but is positioned asneeded depending on whether web tension is to be increased or decreasedas a result of the operation of the web converting machine or the like.

[0012] In a typical application, a dancer is mounted to a pivotablymounted arm to which a positioning force is applied by a spring, asolenoid, a pneumatic cylinder, a hydraulic cylinder, and the like forcegenerator. The positioning force urges the dancer tightly against theweb.

[0013] Often a condition is encountered, e.g., with an out-of-roundunwind roll, that causes cyclical web acceleration followed by webdeceration at a relatively high frequency. As a result, the dancerrapidly pivots back and forth due to a force supplied by the web to movethe mass of the dancer as the web tension cycles between tight andloose. Such rapidly alternating web tension necessarily leads tooperating problems downstream.

[0014] The present invention obviates such problems by simulating a“zero mass” dancer vis-a-vis the web in contact therewith. To that end,a servo motor is utilized in lieu of the conventional force generatorsthat have been used to pivot the dancer arm and tension the web. Anyaction by the web that causes the dancer arm to shift position ismonitored by a position sensor, typically an encoder. Other types ofposition sensors can be utilized as well for this purpose, however. Anoutput signal from the position sensor is transmitted as input to acontroller that signals the servo motor to correct the web tension so asto return the dancer arm to its predetermined “neutral” position.

[0015] The term “servo motor,” as used herein and in the appendedclaims, means an electric, hydraulic or other type of motor that servesas the final control element of an automatic feedback control system formechanical motion.

[0016] Referring to FIG. 1, a web tensioning system 10 is shownutilizing web tensioning device 12 engaging a tensioned segment of web14. Web tensioning device 12 includes dancer arm 16 that carries at thefree end 18 thereof a dancer, such as rotatably mounted dancer roll 20that also serves as a web redirect roller. Fixed end 22 of dancer arm 16is pivotably mounted to base 24 by shaft 26 which is affixed to dancerarm 16 and in turn is journaled in base 24.

[0017] Also fixedly mounted to shaft 26 is an encoder, such asincremental rotary optical encoder 30, which serves as an angularposition sensor and indicates the relative angular position of pivotshaft 26, and thus dancer arm 16. Information from the rotary opticalencoder 30 provides an appropriate input signal to dancer control 34.The encoder can be an incremental encoder or an absolute encoder, asdesired.

[0018] Also operably mounted to pivot shaft 26 is electric servo motor36 which turns pivot shaft 26, and thus pivots dancer arm 16, inresponse to a control output signal from dancer arm controller 34. Inthis manner, initial tension on the web 14 can be set to a predeterminedvalue and thereafter a relatively small rotation of pivot shaft providesinformation via encoder 30 as input signal to controller 34. This inputsignal is processed rapidly by dancer control 34, and a control outputsignal is sent to energize servo motor 36. The control output signaldetermines what torque output and the direction thereof (i.e., clockwiseor counterclockwise) is to be provided by servo motor 36. The amount oftorque to be provided is substantially the same as but opposite to thedancer acceleration or deceleration force, i.e., positive or negativeacceleration force, whatever the case may be. The response time of theservo motor 36 to the control output signal provided by dancer control34 is relatively short, usually of the order of about 0.0005 seconds orless. Thus the torque supplied by servo motor 36 balances out the dancermovement forces as they occur, and web tension remains substantiallyconstant so that the dancer has substantially no inertia to overcome.With respect to the web dynamics, the dancer appears to have “zeromass.”

[0019] The aforedescribed operation of web tensioning system 10 is theso-called feedback control method. A compensating torque commandcomponent is supplied to servo motor 36 in proportion to measuredacceleration or deceleration of the dancer. In any given applicationaccuracy of the feedback control method is determined by the inertia ofthe moving parts of the web tensioning device, accuracy of theacceleration and deceleration measurement, and the informationprocessing rate of the controller.

[0020] Alternatively, a control method can be utilized for webtensioning that depends on web feed rate. In such a case the encoder 30can be omitted, and input to dancer control 34 is provided by a web feedrate actuator in a control loop that includes servo motor 36 and dancercontrol 34. That is, a compensating torque command component is appliedin proportion to web acceleration or deceleration information obtainedfrom the web feed rate actuator.

[0021] The foregoing description and the drawing are illustrative, andare not to be taken as limiting. Still other variations and arrangementsof parts within the spirit and scope of the present invention arepossible and will readily present themselves to those skilled in theart.

I claim:
 1. A web tensioning device which comprises: a base; a dancerarm for engaging the web to be tensioned, having a free end portion witha dancer mounted thereon and a fixed end portion pivotably mounted tothe base; a servo motor operably associated with the dancer arm forpivotally positioning the dancer arm in response to a control outputsignal; and a controller operably associated with the servo motor forgenerating the control output signal in response to a web tensionrequirement.
 2. The web tensioning device in accordance with claim 1wherein the controller generates a control output signal in response tomovement of the dancer arm.
 3. The web tensioning device in accordancewith claim 1 wherein the controller generates a control output signal inresponse to web feed rate.
 4. A web tensioning device which comprises: abase; an angular position sensor; a dancer arm for engaging the web tobe tensioned, having a free end portion with a dancer mounted thereonand a fixed end portion pivotably mounted to the base so as to coactwith the angular position sensor and indicate relative angulardisplacement of the dancer arm as a web in contact with the dancer ismaintained in tension; a servo motor operably associated with the dancerarm for pivotally positioning the dancer arm in response to a controlsignal; and a controller for generating the control output signal inresponse to movement of the dancer arm due to changes in web tension asdetected by the angular position sensor.
 5. The web tensioning device inaccordance with claim 4 wherein the angular position sensor is anencoder operably associated with the fixed end portion of the dancer armand senses relative angular displacement of the dancer arm.
 6. The webtensioning device in accordance with claim 4 wherein the encoder is anincremental rotary optical encoder.
 7. The web tensioning device inaccordance with claim 4 wherein the servo motor is an electric motor.